Originally developed for IBM PC Compatible computers, the Basic Input/Output System (BIOS), also known as the System BIOS, is a de facto standard defining a firmware interface. The BIOS is boot firmware, designed to be the first code run by a PC when powered on. The initial function of the BIOS is to identify, test, and initialize system devices such as the video display card, hard disk, and floppy disk and other hardware. This initialization prepares the machine into a known state, so that software such as an operating system stored on compatible media can be loaded, executed, and given control of the PC. This process is known as booting, or booting up, which is short for bootstrapping.
BIOS programs provide a small library of basic input/output functions that can be called to operate and control the peripherals such as the keyboard, text display functions and so forth. When a system is powered on, BIOS checks system configuration settings and loads programs that act as translators between the hardware components and the operating system into memory. For example, when a user presses a key on the keyboard, the signal is sent to a keyboard interrupt handler, which tells the processor what it is and passes it on to the operating system.
BIOS was developed for the IBM PC at a time when processors functioned in a 16-bit processor mode and addressable memory was limited to one megabyte, and BIOS code reflected IBM PC AT hardware dependencies. Operating systems later developed for 32-bit processors began to include device drivers to handle I/O rather than rely on invoking the 16-bit runtime interface provided by BIOS. These device drivers are often provided by platform firmware and are loaded into memory during BIOS initialization prior to loading the operating system. Because of the large number of peripherals with which an operating system may interface, such platform firmware is often provided by entities other than the system manufacturer. As numerous parties become involved, the distribution of firmware images becomes complicated.
With the proliferation of malware in today's computing environments, the importance of verifying firmware has also increased. A digital signature or digital signature scheme is a mathematical scheme for demonstrating the authenticity of a digital message or document. A valid digital signature gives a recipient reason to believe that the message was created by a known sender, and that it was not altered in transit. Digital signatures are commonly used for software distribution and it is desirable that digital signatures be used for firmware image distribution.